GEO100: Minerals and Rocks Practical Guide and Identification

Comprehensive vocabulary flashcards covering rock-forming minerals, igneous, sedimentary, and metamorphic petrology, including mineral testing criteria, textures, classifications, and laboratory sample standards from GEO100 at NMBU.

Mineral

An inorganic, naturally occurring solid substance with a crystalline (ordered atomic) structure, characteristic physical properties, and a specific, restricted chemical composition (such as $SiO_2$ for quartz).

Rock

An aggregate or solid mixture consisting of one or more minerals, which can also incorporate non-mineral materials such as natural glass or organic matter.

Polymorphism

The phenomenon where minerals have identical chemical compositions but completely different crystalline structures and physical properties due to varying formation pressure and temperature conditions (for example, diamond and graphite, both composed of carbon $C$).

Silicate Tetrahedron

The fundamental building block of all silicate minerals, consisting of a central $Si^{4+}$ ion surrounded symmetrically by four $O^{2-}$ ions to form a $[SiO_4]^{4-}$ tetrahedral group with $109.28^\circ$ bond angles.

Nesosilicates (Isolated Silicates)

A silicate structural subclass consisting of independent, isolated $[SiO_4]^{4-}$ tetrahedrons held together by intermediate cations without sharing oxygen atoms between tetrahedrons; examples include olivine and garnet.

Inosilicates (Chain Silicates)

A silicate subclass featuring tetrahedrons linked into chains by sharing oxygen atoms, occurring either as single chains (such as pyroxene) or double chains (such as amphibole).

Phyllosilicates (Sheet Silicates)

Silicate minerals whose $[SiO_4]^{4-}$ tetrahedrons share three oxygen atoms to form two-dimensional continuous sheets, yielding a distinct platy crystal habit and perfect cleavage in one direction; examples include biotite and muscovite.

Tectosilicates (Framework Silicates)

Three-dimensional networks of silicate tetrahedrons where all four oxygen atoms of every $[SiO_4]^{4-}$ unit are shared with adjacent tetrahedrons, containing no $Mg$ or $Fe$; includes quartz, plagioclase, and K-feldspar.

Mafic Minerals

Dark-coloured silicate minerals rich in magnesium ($Mg$) and iron ($Fe$), including olivine, pyroxene, amphibole, and biotite.

Felsic Minerals

Light-coloured silicate minerals poor in iron and magnesium and enriched in silicon, aluminium, potassium, and sodium; includes quartz, K-feldspar, plagioclase, and muscovite.

Mohs Hardness Scale

A relative, non-linear ten-point scratch-resistance scale ranging from talc ($1$) to diamond ($10$), used to classify minerals as either hard ($> 5$) or soft ($< 5$) using reference materials like a fingernail ($\approx 2$), a knife ($\approx 5$), or a steel nail ($\approx 5.5$).

Cleavage (Minerals)

The tendency of a crystal to split repeatedly along smooth, flat, well-defined planes governed by zones of weak chemical bonding in the crystal lattice.

Conchoidal Fracture

A smooth, curved fracture surface that is typically concave and marked by concentric ripple ridges resembling a bivalve shell; diagnostic of non-cleaving minerals like quartz and volcanic glass.

Streak (Mineral Streak)

The characteristic colour of a mineral in pulverized powder form, tested by dragging the mineral across an unglazed porcelain streak plate.

Acid Test (HCl Reaction)

The application of a drop of $10\%$ hydrochloric acid to detect carbonate minerals such as calcite ($CaCO_3$), which fizzes vigorously by releasing bubbles of carbon dioxide gas via the reaction: $CaCO_3 + 2\,HCl \rightarrow CO_2 + H_2O + 2\,Cl^- + Ca^{2+}$.

Quartz (Kvarts)

A framework silicate ($SiO_2$, NMBU sample Nr. 1) characterized by a vitreous luster, lack of cleavage, conchoidal fracture, and a Mohs hardness of $7$ (scratches glass).

Potassium Feldspar (K-feldspar / K-feltspat)

A light-coloured framework silicate ($KAlSi_3O_8$, NMBU sample Nr. 9) typically displaying a salmon-pink or fleshy reddish-to-white hue, blocky rectangular shape, hardness of $6$, and two cleavage planes meeting at $90^\circ$.

Plagioclase Feldspar (Plagioklas)

A light-coloured framework silicate ($CaAl_2Si_2O_8\text{–}NaAlSi_3O_8$, NMBU sample Nr. 2) exhibiting white, greyish, or transparent coloration, hardness of $6$, two cleavage planes, and delicate parallel striations on cleavage surfaces.

Striations (Plagioclase)

Fine, parallel hair-like grooves visible across cleavage facets that serve as a key diagnostic hallmark distinguishing plagioclase feldspar from potassium feldspar.

Muscovite (Muskovitt / Kråkesølv)

A colourless, silvery sheet silicate ($KAl_2(AlSi_3O_{10})(OH)_2$, NMBU sample Nr. 8) with a hardness of $2\text{–}3$ that peels into thin, elastic, transparent, paper-like flakes due to one perfect cleavage direction.

Biotite (Biotitt)

A dark brown to black sheet silicate ($K(Mg,Fe)_3(AlSi_3O_{10})(OH)_2$, NMBU sample Nr. 6) with a hardness of $2\text{–}3$ and one perfect cleavage direction, allowing individual dark shiny flakes to be peeled away.

Olivine (Olivin)

An isolated silicate ($(Fe,Mg)_2SiO_4$, NMBU sample Nr. 3) characterized by olive-green to yellowish-green color, vitreous luster, a granular or sugary aggregate habit, hardness of $6.5\text{–}7$, and an absence of cleavage.

Pyroxene (Pyroksen)

A single-chain inosilicate ($Ca(Mg,Fe)Si_2O_6$, NMBU sample Nr. 8 mørk) forming dark green to black, stubby or short-prismatic crystals with two prominent cleavage directions intersecting at approximately $90^\circ$ ($87^\circ / 93^\circ$) and hardness of $5\text{–}6$.

Amphibole (Amfibol)

A double-chain inosilicate ($Ca_2(Mg,Fe)_5Si_8O_{22}(OH)_2$, NMBU sample Nr. 7) displaying dark green to black coloration, elongated or needle-like prismatic crystals, hardness of $5\text{–}6$, and two cleavage sets meeting at $60^\circ$ and $120^\circ$.

Calcite (Kalsitt)

A carbonate mineral ($CaCO_3$, NMBU sample Nr. 4 / Unum) characterized by rhombohedral cleavage in three directions, a hardness of $3$ (scratched easily by a pocketknife), and vigorous effervescence in $10\%$ $HCl$.

Garnet (Granat)

An isolated silicate ($(Ca,Fe,Mg,Mn)_3Al_2Si_3O_{12}$, NMBU sample Nr. 10) forming rounded 12-sided dodecahedral ball-shaped crystals, displaying red-brown coloration, no cleavage, and a high hardness of $6\text{–}7$.

Magnetite (Magnetitt)

An iron oxide ore mineral ($Fe_3O_4$, NMBU sample Nr. 5) displaying a metallic to submetallic luster, iron-black color, high density, and strong attraction to a magnet.

Pyrite (Pyritt / Svovelkis / Fool's Gold)

An iron sulphide ore mineral ($FeS_2$, NMBU sample Nederst h.) displaying a brass-yellow metallic luster, high density, hardness of $6$, and well-developed striate-faced cubic crystals.

Chalcopyrite (Kalkopyritt)

A copper-iron sulphide ($CuFeS_2$) with a brass-yellow metallic luster, common iridescent purplish tarnish, high density, and soft hardness ($3\text{–}4$), distinguishing it from pyrite.

Galena (Galena / Blyglans)

A lead sulphide ore mineral ($PbS$, NMBU sample Nr. 12) featuring a silver lead-grey metallic luster, extreme density, cubic crystal habit, and three perfect cleavage planes meeting at $90^\circ$.

Intrusive (Plutonic) Rock

An igneous rock formed beneath the Earth's surface through the slow cooling (thousands to millions of years) of magma, producing medium- to coarse-grained visible crystals ($> 1\,mm$).

Extrusive (Volcanic) Rock

An igneous rock formed at or near the Earth's surface through rapid cooling (hours to days) of lava, yielding fine-grained crystals ($< 1\,mm$) or amorphous volcanic glass.

Phaneritic Texture

A coarse- or medium-grained igneous texture where all individual mineral crystals exceed $1\,mm$ in size and can be readily distinguished with the unaided eye.

Aphanitic Texture

A fine-grained igneous rock texture where the mineral grains are smaller than $1\,mm$, rendering individual crystals impossible to identify without magnification.

Porphyritic Texture

An igneous texture characterized by two distinctly different crystal sizes: large, well-formed crystals called phenocrysts set within a finer-grained matrix or groundmass.

Vesicular Texture

A sponge- or froth-like volcanic rock texture studded with small cavities (vesicles) formed by escaping gas bubbles as magma rises and depressurizes before solidifying.

Obsidian

A dark, highly viscous volcanic glass formed by exceedingly fast cooling (quenching) of felsic lava that prevents crystal lattice formation, exhibiting an amorphous structure and sharp conchoidal fractures.

Colour Index (Igneous)

The percentage estimation of dark (mafic) minerals—namely biotite, amphibole, pyroxene, and olivine—present in an igneous rock, serving as a primary field indicator of chemical silica content.

Leucocratic (Felsic)

An igneous rock containing less than $25\%$ mafic minerals, typically dominated by quartz and feldspars, with a bulk silica content exceeding $65\,wt.\%\,SiO_2$.

Mesocratic (Intermediate)

An igneous rock containing $25\text{–}55\%$ dark (mafic) minerals, possessing an intermediate geochemical silica content of $52\text{–}65\,wt.\%\,SiO_2$.

Melanocratic (Mafic)

A dark-coloured igneous rock containing greater than $55\%$ mafic minerals, corresponding to silica-depleted compositions with less than $52\,wt.\%\,SiO_2$.

Bowen's Reaction Series

A petrological model describing the predictable sequential crystallization temperature of minerals from cooling magma, separating into a discontinuous branch (olivine $\rightarrow$ pyroxene $\rightarrow$ amphibole $\rightarrow$ biotite) and a continuous plagioclase branch (Ca-rich $\rightarrow$ Na-rich), terminating in K-feldspar, muscovite, and quartz.

Granite (Granitt)

A coarse-grained (intrusive, sample I1), leucocratic rock composed predominantly of potassium feldspar, quartz ($> 20\%$), plagioclase, and minor biotite.

Rhyolite (Ryolitt)

A fine-grained (aphanitic, sample I4), light-coloured extrusive volcanic equivalent of granite composed primarily of quartz and K-feldspar phenocrysts in a microcrystalline felsic groundmass.

Granodiorite (Granodioritt)

A coarse-grained intrusive rock (sample I6) with a speckled salt-and-pepper appearance, composed of quartz, plagioclase in greater abundance than K-feldspar, and minor biotite.

Diorite (Dioritt)

A medium- to coarse-grained intermediate intrusive rock (sample I2) displaying a 50/50 balance of light plagioclase and dark amphibole or pyroxene, containing virtually no quartz.

Andesite (Andesitt)

A fine-grained, intermediate extrusive volcanic rock that represents the volcanic equivalent of diorite, typically containing phenocrysts of plagioclase and amphibole.

Gabbro

A coarse-grained, melanocratic intrusive mafic rock (sample I7) dominated by calcium-rich plagioclase, pyroxene, and minor olivine or amphibole, lacking quartz completely.

Basalt

A fine-grained, dark grey to black extrusive volcanic rock (sample I5) representing the volcanic equivalent of gabbro, composed mainly of pyroxene and plagioclase.

Peridotite (Peridotitt)

An ultramafic, coarse-grained intrusive rock representing mantle material, composed almost entirely of olivine and pyroxene with negligible feldspar.

Clastic (Detrital) Sedimentary Rock

A sedimentary rock formed from the physical mechanical breakdown, transport, deposition, and cementation of pre-existing mineral and rock grains (e.g., conglomerate, sandstone, shale).

Conglomerate (Konglomerat)

A coarse-grained detrital sedimentary rock composed of at least $30\%$ rounded to well-rounded pebbles or gravel ($> 2\,mm$) enclosed within a finer-grained sandy or clayey matrix, indicating prolonged transport.

Breccia (Breksje)

A coarse-grained detrital sedimentary rock composed of angular, sharp-edged rock fragments ($> 2\,mm$) cemented in a finer matrix, reflecting minimal transport distance from the sediment source.

Sandstone (Sandstein)

A gritty clastic sedimentary rock composed of sand-sized mineral grains ($0.06\text{–}2\,mm$ / $64\,\mu m\text{–}2\,mm$), commonly made of quartz alongside feldspar or mica.

Arkose

A specific variety of sandstone enriched in sand-sized grains of feldspar alongside quartz, typically formed from the short-distance weathering of granitoid rocks.

Greywacke (Gråvakke)

An immature, dark, poorly sorted sandstone composed of angular sand-sized quartz and feldspar grains set in an abundant, clay-rich matrix.

Shale (Leirskifer)

A fine-grained clastic sedimentary rock composed of clay-sized particles ($< 4\,\mu m$) that splits easily along closely spaced, paper-thin planar layers.

Fossiliferous Limestone (Fossilrik Kalkstein)

An organic chemical sedimentary rock consisting of visible fossil shells and skeletal fragments firmly cemented together in a microcrystalline calcite matrix that effervesces vigorously with $10\%$ $HCl$.

Coquina Limestone

A highly porous, organic chemical sedimentary rock composed of loosely cemented, coarse shell fragments with visible open air cavities.

Chalk (Kritt)

A soft, white, powdery organic chemical limestone made up of microscopic calcite shells of marine microfossils that reacts freely with diluted acid.

Coal (Kull)

A brittle, black organic sedimentary rock with a shiny or dull luster, formed from the heat and compaction of accumulated plant and swamp debris.

Chert (Flint / Kiselstein)

A very hard ($7$), dense, microcrystalline chemical sedimentary rock composed of precipitated silica ($SiO_2$) that displays sharp edges and smooth conchoidal fractures.

Rock Salt (Steinsalt)

An inorganic chemical evaporite rock composed of crystalline halite ($NaCl$), characterized by cubic cleavage, softness, and a salty taste.

Rock Gypsum (Steingips)

An inorganic chemical evaporite rock composed of crystalline gypsum ($CaSO_4$), recognizable by its extreme softness (can be easily scratched with a fingernail, hardness $2$).

Dolostone (Dolomittstein)

An inorganic chemical carbonate rock composed of dolomite ($CaMg(CO_3)_2$) that reacts with diluted hydrochloric acid only after the specimen has been powdered.

Metamorphism

The solid-state transformation of a pre-existing rock (protolith) in response to changes in temperature, pressure, and differential stress without any wholesale melting taking place.

Protolith

The original, unmetamorphosed precursor parent rock from which a given metamorphic rock is generated (e.g., shale is the protolith of slate; limestone is the protolith of marble).

Foliation

A repetitive, sub-planar planar fabric or banded texture in a metamorphic rock created by the preferential parallel alignment of platy or needle-shaped minerals during differential stress.

Slate (Skifer)

A very fine-grained, low-grade foliated metamorphic rock (sample M7) derived from mudstone/shale that splits cleanly into smooth, flat, dull-lustered sheets along slaty cleavage.

Phyllite (Fyllitt)

A low- to medium-grade foliated metamorphic rock (sample M8) derived from mudstone/shale, characterized by a glossy, silky, or silvery sheen and wavy, wrinkled foliation planes caused by fine microscopic micas.

Schist (Glimmerskifer)

A medium- to high-grade foliated metamorphic rock characterized by prominent schistosity, composed of visible, glittering flakes of muscovite and biotite ($1\text{–}5\,mm$) often bearing garnet porphyroblasts.

Gneiss (Gneis)

A high-grade, coarse-grained metamorphic rock (sample M4) displaying alternating light-coloured bands (quartz and feldspar) and dark-coloured bands (biotite and amphibole), producing a characteristic zebra pattern.

Amphibolite (Amfibolitt)

A medium- to high-grade metamorphic rock (sample M3) derived from basalt or gabbro, primarily composed of hornblende amphibole and plagioclase with a weakly foliated, speckled salt-and-pepper texture.

Marble (Marmor)

A non-foliated, crystalline metamorphic rock (sample M5) derived from limestone or dolostone, composed of recrystallized calcite that is soft and reacts vigorously with diluted $HCl$.

Quartzite (Kvartsitt)

A hard, non-foliated metamorphic rock (sample M2) derived from quartz sandstone, composed of interlocked, fused quartz grains with a glassy luster that will not react with acid and scratches glass ($7$).

Hornfels

A dense, hard, very fine-grained, non-foliated contact metamorphic rock (sample M1) formed when country rock is baked by the intense heat of an adjacent igneous intrusion.

Eclogite (Eklogitt / Christmas Rock)

A heavy, high-grade regional metamorphic rock (sample M6) formed under mantle-depth pressures, readily recognized by bright green omphacite pyroxene enclosing deep red garnet crystals.